The present invention relates to a print head used in, for example, a wire matrix printer which performs its printing operation with dots impressed on a printing medium.
A print head of this type comprises an actuator rockable about its own axis and a print wire joined to the free end of the actuator. The actuator is reciprocated to enable the print wire to impress dots on the printing medium so as to perform the printing operation. Depending on the driving system of the actuator, the print head can be classified into, for example, a plunger type, a clapper type and a spring charge type. Included in the spring charge type is a so-called "release-type" print head which comprises a driving means of the actuator consisting of a solenoid, a permanent magnet and a torsion rod. In general, the torsion rod is held in its rest position by the attracting force of the permanent magnet so as to store the energy of torsion spring in the rod. When the printing operation is performed, the solenoid is energized to cancel the attracting force of the permanent magnet, with the result that the energy stored in the torsion rod is released. To be more specific, the actuator is urged in the printing direction by the torsion spring force of the torsion rod. Under the inactive state of the print head, the actuator is held in the rest position by the magnetic force of the permanent magnet, with the result that the torsion energy is stored in the torsion rod. During the printing operation, the magnetic force generated from the solenoid serves to offset the magnetic force of the permanent magnet so as to release the energy of the spring force stored in the torsion rod. The release-type print head outlined above is disclosed in, for example, U.S. Pat. No. 4,167,343 and Japanese patent Disclosure No. 58-8665.
In any type of the print heads described above, the print wire receives the reaction force accompanying the printing impact. As a result, the forward end portion of the print wire tends to be vibrated or bent. To overcome this defect, guide means is mounted to, particularly, the forward end portion of the print wire. In general, a guide member formed of a part of, for example, the casing of the print head is provided with a guide slot so as to provide the guide means. The forward end portion of the print wire is guided through the guide slot to the printing medium, with a small clearance left between the print wire and the wall defining the guide slot.
In the past, it was thought desirable for the print wire not to contact the wall of the guide slot during the normal printing operation. Specifically, the contact of the print wire with the wall of the guide slot was thought to promote wear of the print wire, leading to loss of the operating energy of the print wire. It has been found, however, that a serious defect is brought about if the print wire is not in contact with the wall of the guide slot during the normal printing operation. It should be noted that it is unavoidable for the forward end portion of the print wire to be vibrated by the printing impact. As a result, the printed letters are deformed or made nonuniform as the printing speed is increased. In addition, a lateral external force is given by the printing medium or the ink ribbon to the forward end portion of the print wire, because the print head moves in parallel with the printing medium during the normal printing operation. What should be noted is that the lateral external force mentioned above always acts as a pulling or tensile force during the printing operation so as to cause the joint portion of the print wire to be pulled away from the actuator. As a result, the joint portion tends to be broken easily, leading to a low durability of the print head.